Within the last few years, the following patent applications concerning the preparation of DAS and DAS-salt have been laid open or published. These patent applications show that up to the present DAS and DAS-salt have been prepared by catalytic hydrogenation,
1. in the presence of noble metal catalyst (Japanese Patent Publication No. 815/1973) PA1 2. in the presence of both Pd-catalyst and iodine, bromine or their ions (Japanese Provisional Publication (Kokai) No. 127955/1974) PA1 3. in the presence of a metal in Group 8 as a catalyst (German Provisional Publication (Offenlegungsschrift) No. 2320416) (Japanese Kokai No. 12059/1975) PA1 4. in the presence of MeS.sub.x as a catalyst, where x is an integer in the range 1 - 4, inclusive and Me is a metal in Group 8 or Re (Japanese Kokai No. 93927/1975) PA1 5. on cobalt catalyst (German Offenlegungsschrift No. 2362781) (Japanese Kokai No. 93953/1975) PA1 6. in alcohol solvent, in the presence of nickel catalyst (German Offenlegungsschrift No. 2455394) (Japanese Kokai No. 84550/1975) PA1 7. in the presence of nickel catalyst and in aqueous solution of phosphoric acid, boric acid, organic acids or their salts as buffer materials at pH 5 - 10 (Japanese Kokai No. 84551/1975).
In these patent applications, in (1), (2), (3) and (4) noble metal catalysts are employed for the hydrogenation. The noble metal catalysts are much more expensive as compared with Raney nickel catalyst. It is very difficult to recover the noble metal catalysts in good yield in industrial hydrogenation. This raises problems of unprofitability from an economic standpoint. In comparison, Raney nickel catalyst is inexpensive and advantageous for industrial hydrogenation.
In (3) Group 8 metals are employed as catalysts for the hydrogenation of DNS-salt. In example 19 of (3) Raney nickel catalyst is used for comparison. There an unsatisfactory result is obtained, because much 4,4'-diaminodibenzyl-2,2'-disulfonic acid (hereinafter abbreviated as DABS) is formed as a by-product.
In (4) in general higher pressure is needed for the hydrogenation. This is less advantageous for an industrial process. In Control 4c of (4) Raney nickel is used, but the result is unsatisfactory.
In (5) Raney cobalt catalyst is used. It is more expensive than Raney nickel. Higher temperature and higher pressure are necessary for hydrogenation. Therefore this process is less advantageous.
In (6) alcohol which contains less than 3% of water is used as a solvent. The use of such low-water-content alcohol causes elevation of manufacturing cost as compared with the use of water alone as a solvent.
In (7) Raney nickel is used. But higher temperature is needed. The use of inorganic or organic buffer materials often necessitates complicated treatments clarifying discarded water. Such treatments cause elevation of cost for the hydrogenation. Thus in the prior art the use of Raney nickel catalyst in hydrogenation of DNS or DNS-salt gives unsatisfactory results, because DNS or DNS-salt is hydrogenated at the olefinic bond to some degree simultaneously with the nitro groups. This undesirable side reaction occurs considerably when Raney nickel catalyst is used in these prior-art processes.